Heat exchange recovery method

ABSTRACT

A heat recovery method for at least partially cooling and at least partially cleaning an exhaust stream of hot air from a heating operation which is contaminated with vaporized, condensable liquids and with finely divided particulate solids comprising (a) a first indirect heat exchange means for partially cooling the hot exhaust stream with a fresh stream of cold air to a temperature not significantly below the dew point of vaporized liquids entrained in the hot air stream and for partially heating the fresh stream of air, (b) solids collecting means for centrifugally separating a substantial portion of the finely divided solids from the partially cooled exhaust stream and (c) second indirect heat exchange means for further cooling the exhaust stream to a temperature below the dew point of vaporized liquids initially contained therein to thereby liquefy a significant portion of the vaporized liquids and to further heat the stream of fresh air.

RELATED APPLICATION

This application is a continuation-in-part of copending Paull patentapplication Ser. No. 06/731,525, filed May 8, 1985 and entitled "HEATRECOVERY SYSTEM" now abandoned.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method and system for recovering a portionof the heat normally lost when hot air from a manufacturing operation isvented. More particularly, this invention relates to a heat recoverysystem for at least partially cooling and at least partially cleaning anexhaust stream of hot air from a heating operation which is contaminatedwith vaporized, condensable liquids and with finely divided particulatesolids. In accordance with a preferred embodiment of the presentinvention, (a) a first indirect heat exchange means is provided forpartially cooling the hot exhaust stream with a fresh stream of cold airno lower than a temperature within about 5° F. of the dew point ofvaporized liquids entrained therein and for partially heating the freshstream of air, (b) solids collecting means are provided forcentrifugally separating a substantial portion of the finely dividedsolids from the thus partially cooled exhaust stream and (c) secondindirect heat exchange means is provided for further cooling the exhauststream to a temperature below the dew point of vaporized liquidsinitially contained therein to thereby liquefy a significant portion ofthe vaporized liquids and to further heat the stream of fresh air.Suitable means, such as a variable speed air fan are provided to blowair sequentially over the second and first indirect heat exchange meansat a flow, relative to the temperature, and flow of the exhaust streamof hot air through the first and second indirect heat exchangers, whichis sufficient to achieve the desired degree of cooling in the first andsecond heat exchangers. In the process, the fresh air is heated forsubsequent use (e.g. as air feed for a dryer or a kiln) as it cools theexhaust gas, thus recovering a portion of the heat that would normallybe lost by venting of the exhaust stream.

In accordance with the preferred method of the present invention, hotexhaust gas is diverted away from an exhaust duct through first lateralindirect heat exchange means, next, centrifugally flowed through atransfer chamber and then returned to the exhaust duct through secondlateral indirect heat exchange means. Fresh cool air is simultaneouslycross-flowed across the first and second indirect heat exchange means ata flow correlated with the flow and temperature of the exhaust gas suchthat the exhaust gas is partially cooled to a temperature above the dewpoint of liquids entrained therein in the first heat exchange means, andis further cooled to at least partially liquefy a significant portion ofthe vaporized liquids in the exhaust gas in the second heat exchangemeans, whereby the cool fresh air is heated, whereby the finally dividedsolids in the exhaust gas are selectively centrifugally separated fromthe exhaust gas in the transfer chamber and whereby the condensedliquids separate from the exhaust gas in the second heat exchange means.

1. DESCRIPTION OF THE PRIOR ART

It has heretofore been proposed to preheat the air flowing to amanufacturing operation by bringing it into indirect heat exchangecontact with a stream of hot exhaust air from the operation. Thepresence of finely divided solids and vaporized liquids in the exhaustair always presents a problem, because such materials tend to settle outduring the course of heat exchange thereby impeding flow, or evenblocking flow of the air.

In order to overcome this problem, it has been proposed by the prior artto install a filter in the exhaust line upstream from the indirect heatexchange means in order to remove solids. This is shown, for example, inWinstel U.S. Pat. No. 4,028,817; Briscoe U.S. Pat. No. 4,034,482; TaylorU.S. Pat. No. 4,183,433 and Bullock U.S. Pat. Nos. 4,137,645 and4,204,338. Another relevant patent in this regard is Smith U.S. Pat. No.4, 326,344.

The results obtained with this approach have not been entirelysatisfactory because the filter impedes the flow of air and because thebuild up of solids on the filter surface requires frequent cleaning ifthe apparatus is to function efficiently.

An alternate approach has been to provide a filter downstream of theheat exchange means or to simply exhaust the gas without lint removal asshown, for example, by Schroeder et al. U.S. Pat. No. 4,868,709;McConnell U.S. Pat. No. 4,063,590 and Parker U.S. Pat. No. 4,095,349.

The latter approach is not entirely satisfactory because liquids cancondense out during the passage through the indirect heat exchange meanswhich, together with finely divided solids can impede or block the airflow. Willard U.S. Pat. No. 4,286,528 discloses a filter exhaust systemfor a wood burning stove including a heat exchange unit defining avertical flow path for the exhaust gases which is capable of cooling thegases sufficiently to condense any vaporized creosote. The condensedcreosote flows from the heat exchange unit by gravity.

In Tallman et al. U.S. Pat. No. 4,515,145 a gas-fired, condensing mode,hot air furnace is disclosed wherein a heat exchanger and a condenserare mounted in the exhaust passage for heating fresh air and forremoving water from the products of combustion.

Several embodiments of an exhaust gas treating method are disclosed inWarner U.S. Pat. No. 4,557,202, including an embodiment using twoseparate heat exchangers, wherein incoming air is heated in awater-condensing mode in the first heat exchanger and further heated inthe second heat exchanger.

Briscoe U.S. Pat. No. 4,034,482 is directed to a heating system,including a heat exchanger, wherein exhaust gas from a clothes dryer isintroduced into the return air duct of a building heating system and anexternal bypass connection is provided for the heat exchanger.

2. BACKGROUND OF THE INVENTION

It is common practice in many industries to use driers, ovens, kilns,etc., as part of the manufacturing operation. For example, driers arecommonly used in the textile industry for the heat processing of fabricsand are used in the food processing industry for the spray drying ofedible materials. As another example, in the pulp and paper industry, itis common practice to employ driers in connection with papermanufacturing machinery. Metal fabrication operations frequently employa paint drying oven wherein hot air is used to dry the paint and removesolvents. Driers are used for drying pills and powders in thepharmaceutical industry in the calcining of gypsum, cement and ceramics,and in the curing of plastics and chemicals, etc.

In all of these processes there is a significant loss to themanufacturing operation due to the cost of heating air which isultimately exhausted to the atmosphere.

SUMMARY OF THE INVENTION

The foregoing and related problems are solved in accordance with thepresent invention through the provision of a heat recovery system thatcan be interconnected with the exhaust duct from a manufacturingoperation. The system of the present invention is of a comparativelysimple construction, and with a configuration such that it may beinstalled with ease with a minimum amount of floor space and/or headspace.

In accordance with the present invention, the heat recovery systemcomprises a first heat exchange means, an intermediate solids collectingmeans and a second heat exchange means. This is conveniently provided inaccordance with the preferred embodiment of the present invention byproviding a plurality of elongate heat exchange tubes which are mountedin a generally horizontal (lateral) mode, in parallelism between headplates which are, in turn, secured to a surrounding frame provided withan air inlet and an air outlet to thereby provide a chamber for theindirect heating of a stream of fresh air. In accordance with thisarrangement, the frame containing the heat exchange tubes is laterallymounted adjacent a duct through which contaminated hot air flows in amanner such that the axis of the heat exchange tubes is transverse tothe direction of flow of hot air through the duct. A partitionedtransfer chamber is mounted to the duct-facing end plate of the heatexchange tubes to channel the flow of hot exhaust air into some of thetubes (inlet tubes) of the heat exchanger to provide a first indirectheat exchange means and to channel the flow of partially cooled exhaustair in a manner to be described through the remaining tubes (outlettubes) and back to the duct, to thereby provide a second indirect heatexchange means.

A transfer chamber is mounted to the obverse end plate of the heatexchange bundle so that air flowing through the inlet heat exchangetubes is caused to reverse direction and flow back through the outletheat exchange tubes and to the discharge portion of the transfer chamber

Any suitable means, such as, for example, branch ducts, is provided forchanneling hot exhaust air to the inlet side of the transfer chamber andfor channeling cooled exhaust air from the outlet side of the transferchamber back to the hot air duct. With this construction, appropriatemeans are provided, such as, for example, a bypass damper, forchanneling the flow of exhaust air to the inlet side of the transferchamber and for channeling the flow of cooled exhaust back to the duct.

Appropriate means, such as an air fan, or blower, powered by a variablespeed motor are provided for drawing (inducting) cold air into the heatexchange space defined by the frame, across the heat exchange tubes andout, through the fan to a return line, suitably leading to amanufacturing operation.

Appropriate means, such as a drain are provided for removing condensedliquids from the second heat exchange means.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view, with parts broken away, showing apreferred embodiment of the present invention;

FIG. 2 is a top view, with the top of the frame removed, showing theinterior of the heat recovery system; and

FIG. 3 is a cross-sectional view taken along the line 3--3 of FIG. 2showing the construction of the transfer chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, and especially to FIG. 1, there is shown aheat recovery device A of the present invention operatively connected toa duct B leading from a heating operation (not shown) which contains hotcontaminated exhaust air to be discharged from the system.

As shown in FIG. 1, and also in FIGS. 2 and 3, a plurality of heatexchange tubes 10 are mounted in and supported by a duct side end plate12 and an obverse end plate 14. The heat exchange tubes 10 arepreferably, but not necessarily, o f uniform size and of a diameter suchthat turbulent air flow is encouraged in order to inhibit the settlingof finely divided solids inside the tubes and to enhance the film heattransfer coefficient. In a typical industrial situation, the stream ofhot air flowing through the duct B may be at a temperature of about 200°to about 600° F. and have a velocity of from about 1000 to about 3000cubic feet per minute. In such a situation, tube diameters may rangefrom about 1 to about 3 inches, although larger or smaller diametertubes may be used, if desired. The number of tubes in the tube bundleshould be adequate to handle the amount of air flowing through the ductB without an excessive loss of pressure. Typically, a tube bundle willcontain from about 100 to about 200 tubes. In a specific example of thepreferred embodiment, tubes are two inches in diameter and six feetlong. A total of 156 tubes are used in tube sections E and F, yielding aheat transfer surface area of approximately 500 square feet.

The tube bundle, designated generally by the letter C is encased in aframe 16 of any suitable construction in order to define a shell sideheat exchange space designated generally by the letter D. The frame 16is provided with an inlet opening such as a screened side opening 18 onthe discharge side of the frame and an outlet, such as an outlet port 20(FIG. 2) on the upstream side of the tube bundle C. With thisconstruction, fresh air can flow into the heat exchange space throughthe screened side opening 18, across the heat exchange tubes 10 in theheat exchange space D and out of the frame through the discharge port20. A positive flow of air across the heat exchange tubes can beprovided in any suitable manner such as, for example, through theprovision of an air fan or blower 22 controlled by a variable speedmotor 24 interconnected with the outlet port 20 by means of a suctionside duct 26. The fan 22 is provided with a discharge duct 27 fordelivering the heated fresh air to a desired point in a manufacturingoperation wherein hot air is employed (e.g., a dryer or a kiln).

The frame 16 is supported either from the ceiling or the floor of abuilding in which it is located by any suitable means (not shown) anddisposed so that the longitudinal axis of the heat exchange tubes 10 arelaterally transverse of the axis of flow of hot air through the duct B.Frame 16 is preferably mounted at a slight lateral angle from thehorizontal, sloping downwardly from duct B, such as an angle of about 5°or more, so that condensed liquids will flow more easily toward thedrain 50 for removal from the heat recovery device A.

A transfer chamber 28 is provided for covering the duct side end plate12. The transfer chamber is also provided with a partition or wall 30dividing the tube bundle into a set of inlet tubes E (first indirectheat exchange means) and a set of outlet tubes F. (second indirect heatexchange means). Preferably, the partition is arranged so that an equalnumber of heat exchange tubes are in the incoming section and theoutgoing section. However, this is not absolutely necessary and, ifdesired, the number of tubes in the incoming section can be differentfrom the number of tubes in the outlet section. Access doors 32 and 34are provided in each of the inlet sections and outlet sections forcleaning and maintenance.

Suitable bypass means such as damper 36 are installed in duct B. Aninlet duct 38 is provided interconnecting hot air duct B with the inletside of the transfer chamber 28 and an outlet duct 40 is provideddownstream of the bypass damper 36 for interconnecting the outlet sideof the chamber 28 with the duct B.

An obverse collection chamber 42 is provided which covers the obverseend plate 14 so that incoming air flowing through the heat exchangetubes 10 in the inlet section will reverse direction therein and flowthrough the heat exchange tubes 10 in the outlet section in a returnpath.

In accordance with the present invention, an important function isperformed in the collection chamber 42. Air flowing into the chamber 42must make a 180° turn in order to flow into the outlet tubes. Thecentrifugal effect of the reversing air flow causes substantially all ofthe finely divided particulate solids to be thrown from the flowing airagainst the back wall 44 of the collection chamber. This reversal of thedirection of air flow is significant because it substantially removessolid particulates from the air stream so that there is no significantflow of particulates to outlet tubes F where further heat exchangereduces the temperature to below the dew point of the liquids in the airstream, thereby condensing the liquids so that the condensed liquids canbe removed through the drain 50. This two stage heat recovery method(solids removal followed by liquids removal) substantially prevents themixing together of separated particulates and condensed liquids. Themixing together of separated particulates and condensed liquids tends toresult in the creation of a thick, viscous material which impedes airflow and can even block the tubes.

Therefore the back wall 44 is provided with a trap door 46 to permitperiodic removal of finely divided solids from the obverse chamber 42.

The frame 16 also defines a discharge chamber or outlet box 48 and theframe 16 is preferably mounted so that the obverse end is at a slightlylower elevation than the duct side end. This permits liquids condensedin the tube section F by the cooling action of the incoming fresh air onthe out flowing hot air to collect and run to a drain 50 for dischargefrom the system.

OPERATION OF PREFERRED EMBODIMENT

As an example of the operation of the preferred embodiment of thepresent invention, a stream of hot air contaminated with vaporizedliquids such as water vapor, hydrocarbon solvents, greases, etc., flowsthrough the duct B with the damper 36 open at a flow of about 1000 toabout 5000 feet per minute, such as a flow of 3000 standard cubic feetper minute. In order to practice the present invention, the by passdamper 36 is closed in order to divert the hot contaminated air throughthe inlet duct 38 to the transfer chamber 28 and thence through the heatrecovery device A of the present invention and back to the duct Bthrough the outlet duct 40. The variable speed motor 24 is actuated todrive the air compressor fan at a rate such that fresh air is drawnthrough the screen side opening 18 at a mass flow equal to about 30 to300% of the mass flow of the hot exhaust gas flowing through the duct Band, more preferably, at a mass flow of about 60 to 90% of the mass flowthrough the duct B. The hot, dirty air will be diverted, by closing ofthe damper 36 into the inlet duct 30 and from thence to the inlet side Eof the chamber 28 and from thence through the heat exchange tubes in theincoming section E to the obverse chamber 44 where the direction cf flowwill be reversed causing the air to return by way of the heat exchangertubes in the outlet section F and thence, by way of the duct 40 back tothe main exhaust duct B.

At the same time fresh air at ambient temperature will be drawn acrossthe heat exchange tube of the heat exchange bundle with a cross flowpattern. The fresh air will be at its lowest temperature when it firstpasses across the shell side of the heat exchange tubes in the outletsection F and will be further heated as it flows across the heatexchange tubes in the inlet section E. The thus heated air will bewithdrawn through the outlet 20 and the duct 26 through the fan 22 andthence to the outlet 27 from the discharge side of the air fan 22.

As the incoming hot, dirty air passes through the tube side of the heatexchange tubes 10 in the incoming section, it is partly cooled by thecross flow of air, the cooling being such that the hot air is cooled toa temperature which is preferably above and within 5° F. of the dewpoint of the vaporized liquids contained therein. This is important inorder to eliminate the formation of condensate in the heat exchangetubes in the incoming section E. After flow through the heat exchangetubes 10 in the incoming section E, the hot, partially cooled air entersthe obverse chamber 42 where it reverses direction for flow through theheat exchange tubes in the outlet section F. As a consequence, theparticulate solids are separated from the air stream by centrifugalaction and thrown against the rear wall 42 of the obverse chamber 42 forperiodic collection and removal through the door 46. At this point inthe cooling process, most, and preferably all, of the vaporized liquidswill still be in vapor form. If there is significant liquefication ofthe vaporized liquids prior to this point, the liquids will combinetogether with the finely divided solids resulting in a sticky, difficultto manage mass of material which accumulates on the walls of the inlettubes E.

The partially cooled air from the chamber 42 is further cooled onpassage through the heat exchange tubes E in the outlet section Freaching an appropriate exhaust temperature as it enters the outlet sideof the transfer chamber 28.

As a specific example, if the hot, dirty air entering the duct 38 at2500 standard cubic feet per minute at a temperature of about 250° F.,and if the fresh air coming in through the screen side opening 18 at atemperature of about 70° F., has approximately the same mass flow as thedirty air, then the final temperature of the heated fresh air and of thecooled hot air will be about 170° F. This will result in a recovery ofabout 44% of the heat from the hot, dirty air flowing into the duct 38.Periodic visual inspection of the inside of the tubes E by the operatorwill permit him to determine whether or not the selected predeterminedflow of the incoming fresh air is adequate. Thus, for instance, ifliquids are being condensed in the inlet tubes E, the liquids willcollect with the solids in the collection chamber 42, this will indicatethat the rate of flow of the fresh air is excessive. The operator wouldreduce the flow by appropriate adjustment of the variable speed motor24. As another instance, if the liquids are not condensing in the outlettubes F, this will indicate that further cooling of the exhaust air isdesirable. The operator in this instance would increase the rate of airflow by adjustment of the variable speed motor 24 so as to increase theair flow. However, the rate of flow of the fresh air should not beincreased to an extent such that liquids will again begin to condense inthe inlet tubes E.

Thus, there is no need for an expensive control system for regulatingair flow with the apparatus and method of the present invention,although a control system (not shown) could be used, if desired.

The extent to which the incoming fresh air is heated will be dependentupon the ultimate function for which it is to be used. If the heatedfresh air is to be used as a source of incoming air for a dryer, it willbe desirable to maximize the temperature of the fresh air whereas, asanother example, if the fresh air is to be used for heating a confinedair space such as a work room, a minimal increase in temperature will bedesirable. Therefore, the flow of the incoming fresh air relative to theflow of the hot air in the exhaust duct B may be varied within widelimits, such that the fresh air flow is from about 30% to about 300% ofthe hot air flow, depending, as indicated, upon the degree of heatingrequired. Within these broad flow rates, and also depending upon thetemperature of the air in the exhaust duct B, an absolute temperatureincrease of from about 25° F. to about 200° F. can be obtained bypassage of the fresh air across the heat exchange tubes 10. Usually, theflow of fresh air across the heat exchange tubes 10 will be from about80% to about 150% of the flow of the air through the duct B.

As will be apparent from the foregoing description of a preferredembodiment of the method and apparatus of the present invention, anadvantage of the present invention is the sequential removal, from theexhaust hot air stream, first of particulate solids and second ofcondensed liquids. The normal tendency of vaporized liquids to condenseprior to or during the removal of particulate solids is complicated bythe normal presence in the exhaust gas of vaporized water and oils thatcondense at different temperatures. This problem is resolved inaccordance with the present invention by not cooling the exhaust streamof hot air in the heat exchange tubes E to a temperature that is lowerthan a temperature within about 5° F. of the dew point of the vaporizedliquids.

As pointed out above, the final temperature of the exhaust gas as it isreturned to the duct B from the outlet heat exchange tubes F throughoutlet duct 40 will be determined by the use to which the air dischargedfrom port 27 is to be put (e.g. as the source of air for a dryer or theheater for a work room). This, in turn, will determine the temperatureto which the exhaust air is to be cooled during its passage through theinlet heat exchange tubes E. In general, from about 40% to about 60% ofthe cooling should be done in inlet heat exchange tubes E, provided,however, that the exhaust gas should not be cooled below a temperaturewithin about 5° F. of the dew point of the vaporized liquids in theexhaust gas. This does not present a problem because the operator canvisually inspect the inside of the inlet heat exchange tubes E and theoutlet heat exchange tubes F by looking through opened trap door 46. Theoperator can then take the appropriate corrective action if condensateis undesirably forming in inlet heat exchange tubes E or undesirablyfailing to form in outlet tubes F.

Having thus described my invention, what is claimed is:
 1. A method forcooling and purifying an exhaust stream of hot air flowing through alateral duct and for at least partially heating fresh air, said exhauststream being contaminated with finely divided solids and vaporizedliquids said method comprising the steps of:laterally diverting saidexhaust stream away from said duct through first lateral indirect heatexchange means, comprising lateral heat exchange tubes next laterallycentrifugally flowing said exhaust stream through a transfer chamber andthen laterally returning said exhaust stream to said duct through secondlateral indirect heat exchange means comprising lateral heat exchangetubes, simultaneously laterally cross-flowing fresh air across saidfirst and second indirect heat exchange means in indirect heat exchangecontact with said exhaust stream, setting the rate of flow of said freshair to provide a fresh air flow sufficient to cool said exhaust streamin said first indirect heat exchange means to a temperature not lowerthan a temperature within 5° F. above the dew point of said vaporizedliquids and to cool said exhaust stream in said second indirect heatexchange means to a temperature below the dew point of said vaporizedliquids sufficiently to at least partially liquefy said vaporizedliquids, a. separating and collecting said finely divided solids in saidtransfer chamber for discharge therefrom, b. collecting said liquefiedvaporized liquids condensed within said second indirect heat exchangemeans in said transfer chamber for discharge therefrom, and c.collecting heated fresh air from said first and second indirect heatexchange means for discharge to a heating operation.
 2. A method as inclaim 1 wherein said exhaust stream is flowed into said duct at atemperature of about 200° to about 600° F. at a flow of about 1,000 toabout 3,000 cubic feet per minute, and wherein the fresh air iscross-flowed across said first and second indirect heat exchange meansat an initial temperature of about 50° to about 100° F. and at about 30%to about 300% of the flow of said exhaust stream such that said exhauststream is returned to said duct from said second indirect heat exchangemeans at a temperature of about 110° to about 300° F. and wherein fromabout 40% to about 60% of the total cooling is accomplished in saidfirst lateral heat exchange means.
 3. A method as in claim 2 whereinsaid exhaust stream is flowed into said duct at a temperature of about250° to about 350° F. at a flow of about 1,400 to about 2,100 cubic feetper minute, and wherein the fresh air is cross-flowed across said firstand second indirect heat exchange means at an initial temperature ofabout 50° to about 100° F. and at about 30% to about 300% of the flow ofsaid exhaust stream such that said exhaust stream is returned to saidduct from said second indirect heat exchange means at a temperature ofabout 150° to about 200° F. and wherein from about 40% to about 60% ofthe total cooling is accomplished in said first lateral heat exchangemeans.
 4. A method for cooling and purifying a stream of hot exhaust gasflowing through a lateral duct and contaminated with finely dividedsolids and vaporized liquids emanating from a heating unit and for atleast partially heating fresh air, said method comprising the stepsof:bringing said hot exhaust gas into lateral indirect heat exchangecontact with partially heated fresh air in a first indirect laterallytubed heat exchange zone in order to further heat said partially heatedfresh air and in order to cool said hot exhaust gas to a temperature notlower than a temperature within about 5° F. above the dew point ofvaporized liquids contained threin; then laterally centrifugallyseparating finely divided solid particles from said hot exhaust gas in alateral transfer zone; then bringing said partially cooled exhaust gasinto lateral indirect heat exchange contact with incoming fresh air in asecond indirect laterally tubed heat exchange zone to partially heat thesame to provide the partially heated fresh air for said first indirectheat exchange zone and in order to cool said hot exhaust gas below thedew point of said vaporized liquids contained therein in order to atleast partially liquefy the same; collecting and removing condensedliquids from said second zone; collecting separated finely divided solidparticles in said transfer zone for removal therefrom; returning thecooled hot exhaust gas to said duct; and collecting heated fresh airfrom said first and second indirect heat exchange zones.
 5. A method forcooling an exhaust stream of hot gas contaminated with vaporized,condensable liquids and finely divided particulate solids emanating froma heating operation and flowing through a lateral elongate duct and forheating a stream of fresh cooler air which comprises:a. laterallyflowing said exhaust stream through a first indirect laterally tubedheat exchange means, centrifugally reversing the direction of flow ofsaid exhaust stream in a transfer chamber and then laterally flowingsaid exhaust stream back to said duct through a second laterally tubedheat exchange means, b. simultaneously laterally cross-flowing saidstream of cooler fresh air across said first and second indirect heatindirect heat exchange means in indirect heat-exchange contact with saidexhaust stream, c. setting the flow of said fresh air across said firstand second indirect heat exchange means, relative to the flow andtemperature of said exhaust stream flowing through said first and secondheat exchange means to provide a flow for said fresh air streamsufficient to cool said exhaust stream in said first indirect heatexchange zone to a temperature not lower than a temperature within 5° F.above the dew point of said vaporized liquids therein and sufficient tocool said exhaust stream in said second indirect heat exchange means toa temperature below the dew point of said vaporized liquids, d.separating and collecting said finely divided solids in said transferchamber for discharge therefrom, e. collecting said liquefied vaporizedliquids condensed within said second indirect heat exchange means fordischarge therefrom, and f. collecting heated fresh air from said firstand second indirect heat exchange means for discharge to a heatingoperation.